1. Field of the Invention
The present invention relates to the field of pin grid arrays, specifically the means of making electrical contact between those arrays and a testing device.
2. Prior Art
It is often desired to make contact to the pins of a packaged semiconductor chip in order to test the chip's operability. A contactor is used as the link between the chip and the testing apparatus. In the past these contactors have employed both flexible wire and and sheet metal probes as a means for making electrical connections. The most typical manner of connection is a fixed row of contacts flexed along their length to initiate or disengage contact.
An example of a prior art contactor is shown in FIG. 1. The row of contacts 11 flex along their length in the direction shown. The contacts are mounted in an insulator 12 and coupled to leads 13 which provide connection to the testing device.
There is one disadvantage inherent to prior art contactors, namely high inductance leading to high noise. The prior art attempted to solve this problem through the use of a charge decoupling capacitor. However, the effectiveness of a charge decoupling capacitor diminishes over distance and is not effective if located at a distance of greater than 0.25 inches from the device being tested. The configurations of prior art contactors prevented the locating of capacitors within that distance. When flexible wire contacts are used, the wire length is dictated by mechanical considerations to be approximately one inch, thus any capacitors would be at least one inch from the pins, too far to be effective in reducing noise.
Additional problems that plague prior art contactors are short lifetime, unpredictable contact resistance and a limited temperature range of operating effectiveness.
The short lifetime of prior art contactors (10-20,000 cycles of operation) is due to the fact that the contacts themselves are undergoing dynamic stress. If mechanical failure doesn't occur first, degradation of electrical properties may make the contactor ineffective.
The temperature range of operation for prior art contactors is room temperature to approximately 150.degree. C. It is often desired to make connections at sub-zero temperatures, often as low as -55.degree. C., but past PGA contactors have reduced lifetimes at lower temperatures. This is due to the fact that the ductility of metal is reduced at lower temperatures. Reduced ductility results in a higher failure rate and thus, lower life.
The contact resistance is unpredictable for prior art contactors because the surface to surface contact cannot be controlled. The contact resistance is related to force at application of the contactor divided by area contacted. The force of application decreases with ductility of the contactor and area contacted changes as a result of orientation of the contactor. Flat, flexible metal contactors of the prior art do not consistently present the same force or the same surface area in successive contacts, making contact resistance uncontrollable. This factor, in addition to high noise, may cause the tester to indicate a bad device when the device being tested may actually be good. This reduces yields and causes waste of parts, time and money.
The present invention solves the problems of temperature range, contact resistance, lifetime and the problem of location of decoupling resistors, by a simple, yet effective solution.